Electrochromic devices have important applications as variable transmission windows and eyewear, and as displays for watches, calculators, computers, price signs and updated schedule information in airports and train stations. Electrochromic devices change optical transmittance or reflectance in response to a voltage applied between two terminals on the device. Since the degree of optical modulation is directly proportional to the current flow induced by the applied voltage, electrochromic devices have the capability of continuous tunability of light absorption or reflectance. In addition, such devices exhibit long-term retention of an optical state once achieved, requiring no power consumption to maintain that optical state.
Electrochromic materials variably attenuate light in some region of the electromagnetic spectrum, typically the visible region, on electrochemical oxidation or reduction. The oxidation and reduction reactions must be reversible, as devices are usually required to operate for many switching cycles. Tungsten trioxide (WO.sub.3) is a classic example of an electrochromic material. In an acid electrolyte, WO.sub.3, which is virtually colorless in thin film form, is reduced electrochemically to a deep blue color with the simultaneous insertion of charge compensating hydrogen ions, i.e., EQU xe.sup.- +xH.sup.+ +WO.sub.3 .fwdarw.H.sub.x WO.sub.3
In display applications, the WO.sub.3 is usually deposited as an amorphous (non-crystalline) thin film onto optically transparent, electrically conductive alphanumeric electrode segments. The electrode segments are usually comprised of thin films of optically transparent, electrically conductive oxides such as tin-doped indium oxide (ITO) or fluorine-doped tin oxide (SFO) patterned onto glass. In certain display or mirror applications, metals may be used for some of the thin film electrodes. The deposited WO.sub.3 film is placed in contact with an ion conducting electrolyte, which is in turn in contact with a counter electrode which also undergoes reversible oxidation and reduction reactions. The electrolyte typically contains a pigment against which the optical modulation of the electrochromic layer is contrasted. The pigmented electrolyte is generally opaque so that the counter electrode is masked from view. Typical counter electrode materials in display devices are carbon containing oxidizable and reducible surface groups as taught by Giglia (U.S. Pat. No. 3,819,252, August 1974), or a second film of the electrochromic material as taught by Beegie (U.S. Pat. No. 3,704,057, November 1972).
In electrochromic windows and light modulators for transmittance control, further restrictions on the electrode materials and electrolyte exist. The counter electrode must be, at the very least, transparent to light of the wavelengths being modulated during oxidation and reduction. Examples of such materials are Nb.sub.2 O.sub.5 and TiO.sub.2 (S.Cogan et al., Proc. SPIE, vol 562, (1985), pp. 23-31) or "macroporous" crystalline WO.sub.3 (U.S. Pat. No. 4,278,329, Matsuhiro et al., July 1981). Alternatively, the counter electrode could undergo electrochromic reactions which are complementary to the first electrode, i.e., if the first electrode is colored on reduction, then the counter electrode is colored on oxidation. In this way, the light modulation of the electrochromic electrode and the counter electrode is additive and reinforcing. Examples of electrochromic devices with complementary counter electrodes include: Takahashi et al., U.S. Pat. No. 4,350,414, September 1982; Cogan et al., Proc. SPIE, vol 823, pp. 106-112 (1987); and, Cogan and Rauh, U.S. Pat. No. 5,019,420, January 1992. For most variable transmittance applications, a transparent and colorless electrolyte is preferred to avoid interference with the light modulation.
The ion conducting electrolyte is a very important feature of an electrochromic device. If the electrolyte is a liquid, then, in assembling the device, the electrochromic and counter electrodes may be separated facing each other using a spacer, and the liquid electrolyte injected into the space between them. The cavity may then be sealed, for example, using epoxy. Such seals may leak, however. In order to minimize leakage, gelating agents such as poly(vinyl alcohol) have been added to liquid electrolytes to form semi-solid gels (U.S. Pat. No. 3,708,220, Meyers, January 1973).
Polymeric electrolytes, however, can more effectively alleviate the problem of electrolyte leakage by allowing the fabrication of laminated electrochromic devices in which the polymer has the dual function of ion conducting electrolyte and mechanical adhesive. Such polymers can be formulatedas standalone films with a satisfactory combination of mechanical integrity and ionic conductivity. One class of such electrolytes is the ion-containing polymers known as ionomers. These macromolecules contain ionizable groups covalently linked to a polymer chain, typically a hydrocarbon. Thus, the compounds polystyrene sulfonic acid and poly (2-acrylamido-2-methyl-1-propanesulfonic acid) (polyAMPS) are examples of ionomers, both incorporating the protonic acid -SO.sub.3 H group on the polymer chain. Ionomers are formed by polymerizing monomers bearing the ionizable group and also a C.dbd.C vinylic group, such reactions being initiated, for example, by heat, light or high energy radiation and frequently with the aid of an initiator added as a minor component.
There have been frequent references in prior art to ionomer-based electrolytes for electrochromic devices and for batteries. Giglia et al. (U.S. Pat. No. 4,335,938, June 1982) teaches a polymer electrolyte comprising polyAMPS and H.sub.2 O and further teaches (U.S. Pat. No. 4,375,318, Giglia, January 1983) a homopolymer of polyAMPS with an added organic "humectant" such as polyethylene oxide. Randin (U.S. Pat. No. 4,231,641, November 1980) teaches polystyrene sulfonic acid in electrochromic devices and further teaches (U.S. Pat. No. 4,312,929, Randin, January 1982) a polymeric electrolyte comprising a sulfonic protonic polymer and H.sub.2 O. A discussion of the relationship between polymer composition and performance in electrochromic devices can be found in "Performance Improvements in WO.sub.3 -Based Electrochromic Displays", Proc. SID, vol. 23, p. 1, 1982, by R.D. Giglia and G. Haacke.
In order to function effectively in an electrochromic device over a large number of switching cycles, a polymeric electrolyte should have the following properties:
High oxidative stability. Since electrochromic devices are electrochemical in nature, electrolytes are required which have a high degree of stability at negative and positive polarizations. Prior art concerning co-polymers of vinyl resins and acid-group containing monomers has not addressed oxidative stability in electrochromic devices. Many polymers in prior art are oxidatively unstable. For example, alcohols are readily oxidized in acidic media. An example of an alcohol used in prior art polymeric electrolytes is 2-hydroxyethyl methacrylate (HEMA). Giglia, in U.S. Pat. No. 4,174,152 November 1979, discloses electrochromic devices with binary co-polymer electrolytes prepared from 95-20% of a specified hydroxy alkyl acrylate monomer, of which HEMA is one, and 5-80% of an "acid-group-containing monoethylenically unsaturated monomer". The hydroxy group on the HEMA co-polymer, as demonstrated herein by example, is very susceptible to irreversible oxidation during the operation of an electrochromic device.
Low acidity. It has been demonstrated by Randin (J. Electrochem. Soc., vol 129, 1215 (1982)) that WO.sub.3, a ubiquitous and frequently used electrochromic material, is subject to acid hydrolysis leading to gradual dissolution of the electrochromic film and a decline in optical switching performance. Many prior art polymer formulations, particularly those comprised solely of hydrated sulfonic acid ionomers, were found to be deficient in that they caused excessive dissolution of WO.sub.3 and reduced the useful life of electrochromic devices. Several approaches to improving the stability of WO.sub.3 in acid electrolyte devices were reported. Randin, U.S. Pat. No. 4,312,929 op cit, teaches polymer compositions in which this hydrolysis is partly relieved by employing acidic ionomer electrolytes in which the water is of sufficiently low concentration as to be tightly bound to the anionic groups on the polymer chain. Other methods of minimizing dissolution of the electrochromic layer in contact with the polymer electrolyte include the use of thin layers of less hydrophylic polymers with lower ionic conductivity interposed between the electrochromic layer and the polymer electrolyte (Giglia and Haacke, Proc. SID, op cit) and the use of an inorganic insulator coating on the electrochromic electrode (U.S. Pat. No. 4,193,670, Giglia and Clasen, March 1980).
High mechanical stiffness. Mechanical stiffness differentiates true polymer and plasticized polymer electrolytes from "semisolid" and "gel" electrolytes often referred to in prior art. Mechanically stiff electrolytes are able to support their own weight and retain their shape as stand-alone membranes. They are thus able to act as separators in electrochromic devices. There is frequently a trade-off between mechanical stiffness, controlled by the amount of liquid phase plasticizer, and ionic conductivity. The conductivity increases with increasing plasticizer concentration but the polymer becomes increasingly less viscous, and eventually acquires a gel-like or liquid consistency. In addition, for proton conducting polymers, higher plasticizer concentrations, particularly when the plasticizer is H.sub.2 O, can lead to higher acidities and increased acid hydrolysis of electrochromic materials.
The long-term cyclability of electrochromic devices that employ polymer electrolytes, therefore, depends on achieving oxidative stability and low acidity in a polymer that has the requisite mechanical stiffness to act as a laminant or separator in the device. In addition, the ionic conductivity of the polymer must be high enough that electrochromic devices employing the polymer can switch at speeds fast enough for practical applications.